The overall objective of this research is to define the role of estrogen-induced uterine-, kidney-\and hypothalamic-derived growth factors in the proliferation of rat pituitary tumor cells in vivo and in vitro. The rat pituitary tumor cell line to be used in these studies was established by this laboratory as a subclone of the original GH3 cell population. Our studies with the GH3/C14 cell line have shown that estrogens are required for optimal growth in vivo, but are not capable of promoting continuous growth of these cells in vitro. We have considered several possible explanations for these observations in culture; however, our most recent approach has been to ask whether the mitogenic role of estrogens in vivo may be mediated by steroid hormone control of the production and secretion of growth factors specific for the hormone-responsive pituitary tumor cells. To evaluate the possibility of a mediated mechanism involving estrogen-induced polypeptide growth factor, we first prepared extracts of tissues removed from estrogen-treated and estrogendeficient animals and assayed for growth activity by addition of extracts to cells in serum-free medium. Our experiments demonstrate that estrogens elevate the levels of growth factors for GH3/ C15 pituitary cells in extracts of both rat uterine and rat kidney tissue. We have now purified pituitary tumor cell growth factors from lyophilized powders of sheep uteri and kidneys. These activities are related, but not identical, polypeptides of MW 4,200. These properties are now under further investigation. To date, our experiments suggest a model of hormone-responsive pituitary tumor growth in vivo which includes the mechanism ESTROGENS affect UTERUS, KIDNEY AND HYPOTHALAMUS producing SPECIFIC POLYPEPTIDE GROWTH FACTORS producing ESTROGEN-RESPONSIVE TUMOR CELLS.